The present invention relates to beam formation for free-space optical communication systems. The improved free-space optical communication systems can be used for two-way information transfer between remote objects without any wires and/or optical fibers for connection of these objects, including the case when there are many objects taking part in the information exchange, e.g. for organization of a point-to-multipoint exchange, i.e., a two-way information exchange between a base station transceiver terminal and several subscribers.
Through fibers, optical communications provides high-speed data transmission over relatively long distances, for a wide range of applications and services. The use of fiber, however, is not always practical and/or cost effective. Radio frequency (RF) wireless solutions reduce the time, complexity and cost of installation, but those solutions are inherently limited by their use of shared RF spectrum which is narrow compared to optical spectrum. As the number of users on a given piece of spectrum, the average capacity available to any one user further declines.
Another alternative approach to data communication services involves free-space optical communications. There have been a number of proposals to supply data signals to a laser, couple the laser output to an optical system, transmit the optical signal via line of sight, and recover the information at a remote receiver. Such systems offer two-way information transfer between remote objects without use of wires and/or optical fibers. Because such systems utilize optical radiation characterized by extremely high carrier frequency and can implement non-interfering links to the individual customer premises, such systems are not subject to the limits imposed by the carrier frequency or shared capacity, as in the existing RF and microwave wireless technologies.
In many embodiments of such free-space optical communication systems, the optical radiation from a light source propagates from the transmit terminal to the subscriber from a source of light (in most cases, a laser) with a modulator, driven by a data stream, through a light guide (optical fiber) to an optical antenna (telescope or other optical collector) forming a sufficiently narrow light beam which propagates through free space to a receiving optical system and through another optical fiber to a photodetector. If all other factors are the same, the optical radiation power losses along the above path depend on the geometry (length, diameter, etc.) and type of the optical fibers used.
Free space optical communication systems implemented previously have utilized single mode fibers for transport of the beams from the laser sources to the optical emitting antenna elements. In such an implementation, the signal radiation is guided to the collector by a thin (single-mode) fiber. As known, the radiation passed through such fibers does not have any local minima. Accordingly, there are no such minima in the receive aperture plane (at least if the propagation path has sufficiently high optical quality). An example of such a system is described in Szajowski et al., Eight-channel Video Broadcast Feed Service using Free-Space Optical Wireless Technology at Sydney 2000 Olympic Games, Optical Wireless Communications III, Proceedings of SPIE, Vol. 4214, Nov. 6-7, 2000, pp.1-10.
A drawback of a system with a single-mode fiber is that small diameter of the fiber makes it difficult to obtain high efficiency of the radiation coupling into the fiber from a radiation source, which usually is a laser diode. Commercially available devices comprising a laser diode and a single-mode fiber coupled to it (so-called xe2x80x9cpigtailedxe2x80x9d laser diodes) typically have a radiation coupling efficiency of 25-30%. In other words, the interface between the laser diode and the fiber is actually attenuating the light power by 3-4 times, causing a decrease in the communication range and availability.
Another drawback in using a single-mode fiber is that the light beam formed by it is not resistant to optical inhomogeinities of the free-space optical path. Experiments accomplished by the authors of this invention have demonstrated that, if there are aberrations in the optical path located close to the transmit aperture (rain drops or other small scale aberrations on protecting optical surfaces, such as windows through which the output radiation passes, etc), considerable nonuniformities appear in the transverse intensity distribution of the light on the receive aperture at the remote station. The negative effect of such spatial intensity fluctuations on the quality of communications has already been discussed above. The authors"" experiments proved that the use of a multimode fiber to deliver the optical radiation to the transmitting optical antenna decreases contrast of intensity fluctuations caused by the small-scale optical inhomogeinities located close to the antenna.
Additionally, the manufacturing of the pigtailed laser diodes with single-mode fibers is difficult, and thus the cost of such devices is significantly higher than the costs of the components taken separately, that is to say a laser diode and a fiber applicable for free-space optical communication systems.
The use of a multimode optical fiber, which has a diameter significantly larger than that of a single-mode fiber strongly increases the efficiency of radiation coupling into the fiber, makes alignment substantially easier and considerably reduces the cost of a pigtailed laser diode. However, the multimode fiber creates a different set of technical problems.
In a transmitting system using a fiber with a relatively large core diameter (a multimode fiber), the optical radiation field becomes spatially non-uniform after propagating along the fiber. The radiation field has local maxima and minima in cross-sectional intensity distribution (this is so-called speckle-pattern), with large differences in magnitudes between them. Thereby an optical field with high contrast of the light intensity spatial fluctuations is formed. The fluctuations do not vanish after the radiation passes through the collector; the light beam remains spatially nonuniform along the whole propagation path, including at the receive system aperture of a remote receiving device or system, to which data stream carried by the beam is addressed.
If the receive aperture occasionally coincides with a local optical field intensity minimum, the quality of communication may degrade, which may even break communication because of insufficient received signal power entering the aperture in view of such intensity minimum.
In principle, this effect may be compensated by a manifold increase of the transmitter output power, but such compensation is not practical for technical and cost considerations.
Another way to mitigate the intensity non-uniformity effect is to increase the receive aperture size till it significantly exceeds the average speckle size in the speckle-pattern. In this case the receive aperture always captures several speckles, and the photodetector responds to the optical field intensity averaged over the cross-section of the aperture. However, for a given size of the transmit aperture, when the distance to the receiver system increases, the average speckle size also increases, thus requiring a corresponding increase of the receive aperture diameter, which is not always practical. For a given distance to the receiver, it is also possible to make the speckles size smaller by increasing the transmit aperture diameter. This solution also has the limitations related to: size, weight and cost of transmit optical telescopes (collectors).
In principle, it would be theoretically possible to suppress the fluctuations by spatial decoherentization of the optical radiation, which decoherentization leads to a reduced contrast of interference patterns, including speckle-patterns, created by the optical field. A necessary condition for the decoherentization is the radiation polychromaticity. The polychromatic radiation sources may have various designs. For example, in the patent application EP No.1 12076, 1984, a polychromatic source for use in optical fiber transmission systems comprises several lasers radiating at different wavelengths. However, the radiation polychomaticity provided by this source is insufficient for the decoherentization. A sufficient condition for the decoherentization requires the presence of a path difference between spatial components of the radiation (plane waves in free space, or modes in any waveguide structure along which the radiation is propagating) exceeding the longitudinal coherence length of the radiation, which, as known, is determined by its frequency spectrum width. Perhaps for at least this reason, such sources have not been used to enable multimode fiber transport of signals intended for free-space optical communication.
A method is known of light beam forming, used in point-to-multipoint free-space optical communication systems (see the description of the U.S. Pat. No. 5,786,923). The known method includes modulation of optical radiation by an information signal, and its subsequent concentration on the subscriber""s receiving aperture by an optical system (optical antenna). A disadvantage of the known method is that for its implementation a system is required with the design which is complicated and does not provide for compactness: the light from the modulator is guided to the optical antenna by means of a combination of mirrors, a beam splitter, and a deflector, which in turn requires additional means for alignment of mirrors, or electronic control means (for deflectors).
Another method of light beam forming for free-space optical communication systems, known from U.S. Pat. No. 4,960,315 and U.S. Pat. No. 5,062,150 provides for channeling a modulated optical radiation from a radiation source through a fiber, and its subsequent concentration by an optical antenna.
This method has disadvantages mentioned above. If a single-mode fiber is used in it, then it is impossible to couple all the optical radiation from the source into the fiber because of the small diameter of the fiber. Thus, radiation power losses appear, and a more powerful light source is required. A more powerful light source may comprise a fiber optic amplifier, which is expensive. If a multimode fiber is used, its multiple modes are being excited and interfere inside the fiber, and as a result of their interference a speckle-pattern appears (see, for example, Patent Application EP No. 112076).
At the same time, it is necessary to take into account that a free-space optical communication system design is significantly simplified by way of using fibers for beam delivery in the transmission optical path from the radiation source to the optical antenna (radiation concentrator). It is therefore advisable, on the one hand, to use the fibers in the beam forming systems for free-space optical communication. On the other hand, it is necessary to minimize the radiation losses and the spatial fluctuations caused by the fiber presence in the path between the radiation source and the optical antenna, thus increasing the communication range and availability of the communication channel.
Hence there is an ongoing need for a technique to allow use of a multimode fiber in the beam forming process of a free-space communication system and yet avoid the noted problems with multimode fiber transport.
The optical radiation beam forming method of this invention is oriented towards decreasing below a predetermined level the contrast of the light field speckle-pattern caused by use of multimode fiber in the beam forming process. The use of the multimode fiber reduces power losses in free-space optical communication systems, decreases the system manufacturing costs and increases the communication range and the channel availability. Techniques are provided to design system parameters, at least within ranges, so as to smooth the speckle-pattern and provide a speckle-pattern contrast at or below the predetermined level.
Optical radiation is formed with a frequency spectrum width xcex94v. In an embodiment using a semiconductor laser source, the radiated energy includes a plurality of spectral lines within the frequency spectrum width xcex94v. The optical radiation has been modulated to carry an information signal, which has a frequency bandwidth xcex4v. The modulated optical radiation is applied to a multimode optical fiber, in such a manner that the applied radiation has an angular spectrum width of xcex94xcex8. The angular spectrum width xcex94xcex8 preferably exceeds the diffraction limit angle corresponding to the fiber core diameter of the particular multimode optical fiber. This technique effectively converts the optical radiation into the complete set of multimode fiber modes supported by the fiber. Within the fiber, the maximum propagation velocity difference between the said modes being xcex94V.
Distinguishing features of the inventive technique include the determination of the parameters of the optical radiation and the fiber parameters based on the following expressions:                     L        ·        Δ            ⁢              xe2x80x83            ⁢      v         greater than                   V        2                    Δ        ⁢                  xe2x80x83                ⁢        V                  a    ⁢          xe2x80x83        ⁢    n    ⁢          xe2x80x83        ⁢    d                      L        ·        δ            ⁢              xe2x80x83            ⁢      v         less than                   V        2                    Δ        ⁢                  xe2x80x83                ⁢        V            
where:
L is the fiber length, m;
xcex94v is the frequency spectrum width of the formed optical radiation, secxe2x88x921;
xcex4v is the modulation signal frequency bandwidth, secxe2x88x921;
V is the mean velocity of the optical radiation modes propagating along the fiber, m/sec;
xcex94V is the maximum difference between propagation velocities of the modes exited by the radiation in the fiber, m/sec, which difference depends on the fiber material refraction index distribution across the fiber and on the angular spectrum width xcex94xcex8; and
xcex94xcex8 is the angular spectrum width of the optical radiation coupled into the fiber input end, measured in radians.
The first expression             L      ·      Δ        ⁢          xe2x80x83        ⁢    v     greater than             V      2              Δ      ⁢              xe2x80x83            ⁢      V      
serves to define the minimum parameters for the necessary incoherence, that is to say the minimum requirement that the spectral components of the radiation at the outer boundaries of the frequency spectrum width xcex94v are incoherent with respect to each other at the fiber output.
As an optical radiation carries information through a multimode fiber characterized by intermodal dispersion, the longer the distance traveled the more the propagation tends to overlap bit intervals because of different velocities of the modes in the fiber (for example, in the case of amplitude modulation, intermodal dispersion distorts the information carrying pulses). The second expression             L      ·      δ        ⁢          xe2x80x83        ⁢    v     less than             V      2              Δ      ⁢              xe2x80x83            ⁢      V      
serves to define a maximum limit on the fiber length, so that the fiber is not so long as to overly degrade the information transmission due to pulse lengthening or the like as the radiation traverses the length of the fiber.
Another aspect of the inventive methodology involves forming the optical radiation frequency spectrum of separate spectrum lines with a minimum spacing between them, exceeding xcex4v.
Another feature of the method relates to forming the optical radiation with a number of spectrum lines N and an angular spectrum width xcex94xcex8 providing excitation of A modes within the fiber, while N and A are selected in accord with the following expression:                     A        +        N        -        1                    A        ·        N              ≤          C      2        ,
where:
N is the number of the radiation spectrum lines;
A is the number of modes excited in the fiber at each of the spectrum lines; and
C is a targeted limit for maximum contrast of the optical radiation field speckle-pattern at the fiber output.
Maximal speckle-pattern contrast is lower than C, provided the inequality             A      +      N      -      1              A      ·      N        ≤      C    2  
is satisfied, and each of the optical fields corresponding to each of the N radiation spectrum lines and consisting of A modes are incoherent with respect to each other. To provide such incoherence it is necessary to pre select the parameters of the optical radiation and of the fiber so as to satisfy the expression:             L      ·      Δ        ⁢          xe2x80x83        ⁢    v     greater than             (              N        -        1            )        ·                            V          2                          Δ          ⁢                      xe2x80x83                    ⁢          V                    .      
Using the above rules of selection the radiation and the fiber parameters, one can decrease the speckle pattern contrast below value C permitted by the free-space optical communication system design and avoid the negative effect of speckle-patterns on the system operation.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the accompanying drawings or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.